Retrospective measurements underestimate nightmare frequency, but little is known about how retrospective duration and attention for nightmares may affect this process. This study evaluates the differences between two retrospective durations, a prospective log, and a retrospective estimate after keeping this log. Forty-nine participants completed the SLEEP-50, kept a nightmare log, ... read more and estimated their nightmares after keeping a log. Paired t-tests showed that estimates of nightmare frequencies differed significantly from each other according to measurement type (p < .05). Prospective logs are the most accurate way of estimating nightmare frequency, possibly due to simply forgetting over time. For treatment studies relying solely on retrospective measurements, a short duration is recommended. show less